It was first attributed to the Russians by CrowdStrike. I have yet to see any even remote indication of partisanship, especially pro-Democratic partisanship by CrowdStrike or its ownership.
CrowdStrike's partisanship is like the news media's: bias towards spectacle, unwarranted certainty, and producing a coherent narrative facts be damned.
They don't get paid for the accuracy of their public pronouncements, it's just a marketing tool to raise their profile. It doesn't prevent them from being competent and maintaining clients.
Also, most clients would be more than happy to have someone proclaim that the reason they got owned was because they were the target of spooky foreign state actors and not because they're bad at security. There's no money in downplaying the sophistication of an attack.
But the biggest indicator is their attribution based on internally coherent narratives instead of facts - that's not how actual digital forensic experts operate.